(1) Field of the Invention
The present invention relates to ceramic heaters to be used in various semiconductor-producing apparatuses, etching apparatuses, etc.
(2) Related Art Statement
NGK Insulators, Ltd. disclosed a ceramic heater in which a wire made of a metal having a high melting point is buried in a discoid substrate made of a dense ceramic material. This wire is spirally wound inside the discoid substrate, and terminals are connected to both ends of the wire. It was found that such a ceramic heater has excellent characteristics particularly for producing semiconductors. However, this ceramic heater is produced as follows. First, a wire made of the high melting point metal is spirally wound, terminals (electrodes) are attached to both the ends of the wire, and they are annealed in vacuum. On the other hand, a powdery ceramic material is charged inside a press-molding machine, and preliminarily molded to a given hardness, while a depression is formed in a surface of the preliminarily molded body. The above wire is accommodated in the depression, and the ceramic powder is further charged onto the resultant. Thereafter, the resulting powdery assembly is uniaxially press molded to a discoid molded body, and the discoid molded body is sintered by hot press.
However, it is very difficult to carry the resistance heating member from an annealing apparatus to the preliminarily molded body without breaking the shape of the resistance heating element, so that the shape is often unavoidably broken. Further, after the resistance heating element is placed in the depression of the preliminarily molded body, the ceramic powder is filled on the preliminarily body, followed by the uniaxial press molding. However, since the charged density of the powder locally varies, the shape of the resistance-heating element is likely to be broken at that time.
In order to solve the above problems, NGK Insulators, Ltd. proposed in JP-A-5-275434 a method that a metallic foil is placed on a preliminarily molded body, ceramic powder is charged onto the preliminarily molded body, and a discoid molded body is produced by uniaxially press molding the resulting ceramic powdery assembly. According to this method, since the resistance-heating element is made of the metallic foil, which does not deform three-dimensionally different from the wire, the resistance-heating element loses its shape during carrying or placing it. JP-A 6-260263 proposed that a ceramic heater in which a foil-shaped resistor is buried inside a dense ceramic substrate is produced by first preparing a plurality of ceramic shaped bodies by cold isostatic press, laminating the ceramic shaped bodies while placing the foil-shaped resistor between the ceramic shaped bodies, and sintering the laminate by hot press.
The present inventors advanced investigations upon various ceramic heaters, and proceeded with development to decrease the thickness of ceramic heaters. During this, it was found that the substrate could be made thinner in the ceramic heater having the above foil-shaped resistance heating element buried in the dense ceramic substrate than in the ceramic heater having the linear resistance heating element buried therein. However, it was found that the following new problem existed in the heater having the foil-shaped resistance-heating element buried in the ceramic substrate. That is, when ceramic heaters were repeatedly subjected to heat cycles at a number of times in which the ceramic heater was operated at not less than 300.degree. C., e.g., in a high temperature range 300 to 100.degree. C., and then cooled to a temperature range of not more than 100.degree. C., some of the ceramic substrates were partially cracked.